The present invention relates to technique of detecting a failure of an engaging element in a step automatic transmission, and technique of a fail-safe control in case of such a failure.
A step automatic transmission includes a planetary gear mechanism and a plurality of friction engaging elements or devices to select a desired gearing or gear position to provide a desired speed or gear ratio by changing over between an engaged state and a disengaged or released state. Among various friction engaging elements, there is such a friction engaging element that a required engagement capacity varies widely in dependence on the gearing position. If a hydraulic circuit is to change a hydraulic fluid pressure supplied to such a friction engaging element, to meet required engagement capacities differing largely in dependence on the gear speed, then the hydraulic pressure control becomes very complicated.
Therefore, a Japanese patent document JP 05-288264A discloses technique of preventing deterioration of a shift shock by employing a friction engaging element formed with two pressure chambers having different pressure receiving surfaces, and a hydraulic circuit to supply an oil pressure to one or both of the pressure chambers in accordance with the required engagement capacity.